Human infection patterns and heterogeneous exposure in river blindness

doi:10.1073/pnas.0502659102

Human infection patterns and heterogeneous exposure in river blindness

Departments of ^*Infectious Disease Epidemiology and ^‡‡Epidemiology and Public Health, Imperial College London, Norfolk Place, London W2 1PG, United Kingdom; ^‡Institut de Recherche pour le Développement, 213, Rue La Fayette, Paris Cedex 10, France; ^§Institut für Tierphysiologie, AG Parasitologie, Universität Tübingen, Friedhofstrasse 73, D-72074 Tübingen, Germany; ^¶P.O. Box 715, Sonoita, AZ 85637; ^∥Departamento de Salud Preventiva y Social, Facultad de Medicina, and ^††Laboratorio de Biología de Vectores, Instituto de Zoología Tropical, Facultad de Ciencias, Universidad Central de Venezuela, Aparto Postal 47072, Caracas 1041-A, Venezuela; and ^**Centro Amazónico para Investigación y Control de Enfermedades Tropicales, Aparto Postal 59, Puerto Ayacucho 7101, Amazonas, Venezuela

Edited by David Cox, University of Oxford, Oxford, United Kingdom (received for review April 1, 2005)

Abstract

Here we analyze patterns of human infection with Onchocerca volvulus (the cause of river blindness) in different continents and ecologies. In contrast with some geohelminths and schistosome parasites whose worm burdens typically exhibit a humped pattern with host age, patterns of O. volvulus infection vary markedly with locality. To test the hypothesis that such differences are partly due to heterogeneity in exposure to vector bites, we develop an age- and sex-structured model for intensity of infection, with parasite regulation within humans and vectors. The model is fitted to microfilarial data from savannah villages of northern Cameroon, coffee fincas of central Guatemala, and forest-dwelling communities of southern Venezuela that were recorded before introducing ivermectin treatment. Estimates of transmission and infection loads are compared with entomological and epidemiological field data. Host age- and sex-heterogeneous exposure largely explains locale-specific infection patterns in onchocerciasis (whereas acquired protective immunity has been invoked for other helminth infections). The basic reproductive number,R ₀, ranges from 5 to 8, which is slightly above estimates for other helminth parasites but well below previously presented values.

Footnotes

↵ † To whom correspondence should be addressed. E-mail: joao.filipe{at}lshtm.ac.uk.
Author contributions: M.-G.B. designed research; J.A.N.F. and M.-G.B. performed research; J.A.N.F. contributed new analytic tools; J.A.N.F. analyzed data; J.A.N.F. and M.-G.B. wrote the paper; M.B. collected data in Cameroon and gave general advice; A.R. collected data in Cameroon; R.C.C. collected data in Guatemala; S.V.-M., M.-E.G., and M.-G.B. collected data in Venezuela; and M.P.L. gave statistical and general advice.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: mf, microfilariae; L3, third-stage infective larvae; ABR, annual vector biting rate; C.I., confidence interval; NB, negative binomial.